Grease Pencil : How to prevent rounded stroke ends
Since Blender Version 2.92 Alpha and onward (download V2.92) you can use the cutter tool to remove the caps of your lines.
From wiki.blender.org :

Patch note : https://developer.blender.org/rB2985a745bb01
Also there is an option to automerge strokes ends while drawing, in which you could be interested.

Disabled :

Enabled :

Patch note :
https://developer.blender.org/rBe9607f45d85d